1. Field of the Invention
The invention relates to an article of manufacture comprising: a substrate composed of a superalloy containing chromium and a base element selected from the group consisting of iron, cobalt, and nickel; and an enrichment layer containing chromium and placed on the substrate.
The invention also relates to a method of manufacturing an article comprising: a substrate composed of a superalloy containing chromium and a base element selected from the group consisting of iron, cobalt, and nickel; and an enrichment layer containing chromium and placed on the substrate; wherein the enrichment layer is placed by precipitating chromium onto the substrate and diffusing precipitated chromium into the substrate to form the enrichment layer.
The invention further relates to a method of manufacturing an article comprising a substrate composed of a superalloy containing chromium, a base element selected from the group consisting of iron, cobalt, and nickel, and a combining element which forms a gamma-prime phase intermetallic compound with the base element and an oxide scale as subjected to an oxidizing condition at a high temperature; and an enrichment layer containing chromium and placed on the substrate; wherein the enrichment layer is placed by precipitating chromium onto the substrate, diffusing precipitated chromium into the substrate to form the enrichment layer and diffusing the combining element from the substrate into the enrichment layer.
2. Description of the Pertinent Art
An article of this type and methods of these types are apparent from the book "Superalloy II", edited by C. T. Sims, N. S. Stoloff and W. C. Hagel, John Wiley & Sons, New York 1987. Of particular relevance in this context are chapter 4 "Nickel-Base Alloys", pages 97 ff., chapter 5 "Cobalt-Base Alloys", pages 137 ff., and chapter 13 "Protective Coatings", pages 359 ff.
The book also contains an extensive survey of the whole technical field of nickel-base and cobalt-base superalloys, their manufacture, and their application in heat engines, in particular stationary and mobile gas turbines.
U.S. Pat. No. 5,499,905 relates to a metallic component of a gas turbine installation having protective coating layers, wherein the component is formed of a nickel-base base material and at least two coating layers, which coating layers are optimized to resist corrosive attacks within specified temperature ranges. The coating layers may include an inner layer in the form of a diffusion layer formed by diffusing chromium into the base material. Another coating layer formed of an alloy of the type MCrAlY, composed of a metal M selected from iron, cobalt, and nickel, further chromium, aluminum and yttrium or another rare earth metal. Further ingredients, including rhenium, may also be present.
WO 93/03201 A1 relates to the refurbishing of corroded superalloy or heat resistant steel parts and parts so refurbished. During the refurbishing, corroded superalloy or heat resistant steel parts like gas turbine components are stripped of products of corrosion and damaged protective coatings eventually present, and may be provided with new protective coatings. Such a protective coating can be formed by diffusing chromium into the refurbished part, or by applying an MCrAlY-type alloy, inter alia.
U.S. Pat. No. 5,401,307 relates to a high temperature-resistant corrosion protective coating on a component, in particular a gas turbine component. The component is in particular formed of a nickel-base of cobalt-base superalloy, and the corrosion protective coating is composed of a specially developed MCrAlY-type alloy. That alloy is also very suitable to bond a ceramic thermal barrier layer to the component.
In this context, U.S. Pat. No. 5,262,245 describes an effort to modify a nickel-base superalloy to make it suitable to anchor a ceramic thermal barrier layer directly on a thin, adherent alumina scale formed on the superalloy.
Meanwhile, modifications to MCrAlY-type alloys and superalloys have been proposed which include replacing aluminum partly or wholly by gallium. In this respect, it is expected that gallium retains corrosion-protective and structurally relevant features of aluminum but avoids an embrittlement which must be expected if the proportion of aluminum in a respective alloy is increased.
WO 96/34130 A1 concerns a superalloy article which is hollow and thereby has an outer side to be exposed to a hot flue gas during service and an inner side to be exposed to a cooling gas like compressed air or steam. To provide oxidation and corrosion resistant properties for the inner side of the article, the inner side has an aluminide coating. This aluminide coating is made by precipitating aluminum onto the inner side and diffusing the aluminum into the superalloy. In that context, it will not generally be possible to avoid a concurrent precipitation of aluminum onto the outer side of the article, which outer side is subsequently to be provided with another protective coating. To avoid problems which might result from the embrittling property of the aluminum on the outer side of the article, a special manufacturing method as well as an article so manufactured are shown.
A nickel-base superalloy can be characterized in general terms to comprise a continuous matrix composed of a gamma-phase solid solution of chromium in nickel and a precipitate granularly dispersed in and coherent with the matrix and composed of a gamma-prime-phase intermetallic compound formed of nickel and aluminum and/or titanium. In the following text, elements like aluminum and titanium are termed "combining elements". To specify the precipitate as coherent with the matrix means that crystalline structures of the matrix are continued into the grains of the precipitate.
Thus, there are generally no cuts or cleavages between the matrix and the grains of the precipitate. Instead, an interface between the matrix and the grain of the precipitate is characterized by a local change in chemical composition through a continuous, however strained, crystal lattice. Further precipitates generally not coherent with the matrix may also be present. These further precipitates include carbides and borides. Also, additional elements are generally present in the superalloy, and these elements must be expected to be distributed in the matrix as well as in the precipitate. These additional elements may comprise elements which have a particular high affinity to form the said further precipitates like carbides and borides. Elements of this type are niobium, tungsten, hafnium and zirconium.
A cobalt-base superalloy can be characterized in general terms to comprise a continuous matrix composed of a gamma-phase solid solution of chromium in cobalt. This continuous matrix can generally be strengthened by various alloying elements, and precipitates granularly dispersed in the matrix and formed of compounds like carbides and borides can generally be present as well. In contrast to nickel, however, cobalt does not form a gamma-prime phase compound with aluminum or titanium which could serve as a principal strengthening component. As compared to nickel-base superalloys, cobalt-base superalloys are generally inferior with regard to strength; but cobalt-base superalloys are superior as regards thermal stability. Accordingly, both nickel-base alloys and cobalt-base alloys are applied in gas turbine industry. In general, nickel-base alloys are utilized for highly stressed moving components like first-stage gas turbine blades, whereas cobalt-base superalloys are utilized for components under extreme thermal but moderate mechanical stress like first-stage gas turbine vanes.
Recent efforts to improve creep rupture properties of nickel-base superalloys have resulted in alloys wherein the proportion of the intermetallic precipitate amounts up to 50% in parts by volume and even more. Thereby, these alloys have superior creep properties at temperatures above 750.degree. C. However, an increase of the proportion of the intermetallic precipitate must be met by a decrease of the amount of chromium in the superalloy, since chromium is predominantly concentrated in the matrix and hardly stored in the precipitate. However, chromium is a major promoter of oxidation and corrosion resistance of the superalloy, as chromium shows an effect of promoting diffusion of aluminum, and presumably also gallium, to form an aluminum or gallium oxide scale on the superalloy under suitable conditions. Accordingly, a reduction of chromium in a superalloy must generally be expected to be followed by a decrease in corrosion and oxidation resistance, which contravenes of course pertinent interests, even if only to avoid immediate failure of a superalloy component if its protective coating has received some kind of damage.
In cobalt-base superalloys, the strengthening effect obtained by forming a coherent precipitate of a gamma-prime compound is much less pronounced than in nickel-base superalloys. Cobalt-base superalloys generally rely on solid solution strengthening effects obtained by alloying elements which form a gamma phase solid solution with cobalt. Additionally, non-coherent precipitates like carbides and borides may be utilized. However, it may be advantageous to form precipitates of intermetallic compounds formed with aluminum, in particular, even if only to utilize the corrosion and oxidation protective properties of aluminum, as explained for nickel-base superalloys. With regard to these properties, the element chromium, which is generally present in a cobalt-base superalloy, also plays a promotive role, as explained for nickel-base superalloys. Much as for nickel-base superalloys, it might be desirable to keep the chromium content of a cobalt-base superalloy predominantly low in order to obtain certain benefits with regard to structural properties and yet retain oxidation and corrosion resistant properties which usually require a chromium content above a certain limit.
A diffused chromium-containing layer on a superalloy substrate may be termed "enrichment layer" as characterized by an enrichment of chromium. Such a diffused layer can generally have a concentration gradient of chromium increasing from a minimum value substantially equal to a concentration of chromium in the substrate at an interface between the substrate and the enrichment layer to a maximum value greater than the minimum value at a surface of the enrichment layer facing away from the substrate. This is of course due to the diffusion process itself used to form the layer. The enrichment layer generally has a predominantly high concentration of chromium at its outer surface. Thereby, so-called alpha-phase chromium compounds which are characterized by a body-centered cubic crystal structure occur at least at and/or near the surface. If exposed to an oxidizing condition at a sufficiently high temperature, the enrichment layer is expected to form a chromium oxide scale on its surface, which scale is expected to suppress any further oxidation of the enrichment layer or the substrate. If aluminum or another combining element is present in the enrichment layer, it may be expected to be stored in beta-phase compounds like NiAl. From these compounds, the combining element may diffuse to the surface of the enrichment layer and form an oxide scale of its own oxide in addition to, or in replacement of, the chromium oxide scale under suitable conditions.
In practice, it has been observed that chromium-containing layers formed on superalloy articles are prone to rapid degradation if exposed to oxidizing and corrosive conditions as occur during usual service. Accordingly, aluminide layers formed by diffusing aluminum into superalloy articles have attained a widespread use for protective purposes, while accepting the brittleness of the aluminides usually formed. The major problem however eventually resulting from the brittleness is a tendency for cracking under mechanical load.